The present invention relates to a force element arrangement in relation to a riser joint and method for reducing bending moment in a riser at the connection point to a subsea installation, and more specifically to a riser having a flexible joint.
During subsea hydrocarbon extraction a riser is utilized to establish a conduit between a floating vessel and a subsea wellhead. Due to the fact that the riser at one end is fixed to the structure on the seabed and at the other end to a vessel that is under the influence of wind and waves, the riser is exhibiting stresses as the vessel moves. The riser is held in tension from the vessel and this will result in bending stresses in the riser as the vessel moves. To minimize these bending stresses the riser is equipped with a flex joint and or possibly a bend restrictor at the wellhead. A bend restrictor will resist bending and avoid point stresses at the connector, but will not reduce the bending moment as such. An example of a flex joint as used in the industry is shown in U.S. Pat. No. 5,951,061. Such a joint is designed with a certain stiffness to resist bending and, when bending occurs, to force realignment of the riser back to a neutral position.
A constant bending stress in itself will normally not damage the wellhead since the connector and the wellhead is designed to withstand these forces. However, the bending may be cyclic, due to vessel movements, and these cycles may result in fatigue problems at the wellhead.
In FIG. 1 there is shown a prior art riser system for use in well completions and workover operations. A well 10 has been drilled from the seabed 12 into the earth and completed in the normal manner, capped with a wellhead and subsea Christmas tree 14. A BOP or lower riser package (LRP) 16 is locked onto the Christmas tree 14. An emergency disconnect (EDP) 18 is locked to the LRP. Above the EDP there is normally arranged a stress joint 20 that will handle bending moments in the riser. The stress joint 20 may be in the form of a bending restrictor. At the lower end of the riser there is also a safety joint or weak link 22. The riser 24 itself consists of a number of pipes that are screwed or otherwise locked together to form a pipe string as is well known in the art. At the top of the riser there is a telescopic joint 26. In the drawing the telescopic joint is shown in its collapsed position. The riser 24 is held in tension using a tension system 28 in the normal manner. A surface flow tree is attached to the top of the riser and held in tension using the heave compensator (not shown). The vessel has a cellar deck 32 and a drill floor 34. All operations are conducted on the drill floor.